1. Field of the Invention
The present invention relates generally to the field of electrohydraulic servovalves, and more particularly to servovalves having high dynamic response in which a spool may be commanded to move at high velocities relative to a surrounding sleeve, bushing, or body.
2. Description of the Prior Art
It is known to provide a two-stage electrohydraulic servovalve with second-stage sliding spool, the position of which relative to a body or sleeve modulates flow through the valve's hydraulic control section. Four examples of such a servovalve are shown in U.S. Pat. Nos. 3,023,782, 3,103,739, 3,228,423, and 3,257,911. The several disclosures of which are hereby incorporated by reference.
While their outward configurations are known, the spool member and the cooperative member in which the bore is provided, have both been typically formed of a relatively hard material, such as 440C stainless steel. Such members have also been formed of a less hard material, to facilitate machining, and then subjected to a suitable surface hardening treatment. Such hard material and surface treatment, have been deliberately employed so that the flow metering edges and lands on the spool and bore members, would be hard and long lasting.
Servovalves have found use in an ever-increasing number of applications. In most applications, provision of a "hard" spool and "hard" mating surfaces on the bore member, is completely acceptable.
However, in other high performance applications, typically where the spool is required to move relative to the sleeve with peak velocities on the order of 40 inches per second or greater, the bearing surfaces on the spool and bore member have a tendency to chafe or gall. In such use, conventional servovalves have been found to have an unpredictable life, and often fail by a "stuck" spool which has welded itself to the bore member. Upon examination, it has been noticed that such welding usually occurs in the vicinity of the bearing lands, which remain in metal-to-metal engagement with the bore member throughout the limits of spool motion association with normal servovalve operation.
As a practical matter, contaminant particles are always present in the hydraulic fluid being controlled. It is felt that valve failure may be caused, at least in part, by the presence of micrometer-size contaminant between the bearing surfaces of the spool and bore members. Also it is felt that such failure is aggravated by side loading of the spool bearing lands from nonconcentric spool centering springs, such as may exist in FIG. 1 of U.S. Pat. No. 3,228,423, or from non-concentric spool end driving surfaces such as may be created in FIG. 7 of U.S. Pat. No. 3,257,911 should the reduced diameter spool sections be formed by individual cylindrical members.